texts: How far apart are stars in Terzan 5 cluster?

texts:
I've just learned about the Terzan 5 globular cluster, which according to Wikipedia has one of the highest star densities in our Galaxy - $10^6\ M_\odot/\mathrm{pc}^3$.

Problem is, I've no idea how to interpret this number.

So, the question is, what are the distances between stars in that cluster on average?


texts: Is the Higgs field found everywhere? Is it responsible for our mass, and are we in the Higgs field?


texts: Terminology question: gas giant vs gas planet

texts:
While not exactly the most exciting question, I'm wondering: is there any real, semantic difference between a gas planet and gas giant, or are the two terms used interchangeably by most in popular usage (i.e. popular-scientific texts)?

now, a gas planet is obviously the umbrella term for all planets composed mainly of gas, but I often see "gas planet" and "gas giant" used interchangeably without any real change in meaning. I'd class Neptune & Uranus as "ice giants" and Jupiter & Saturnus as "gas giants", but (popular) dictionaries like https://www.thefreedictionary.com/ ; https://en.oxforddictionaries.com/ still seem to group them all into gas giants.


texts: What does Eddington Limit (Eddington Luminosity) Show?

texts:
We know Eddington Limit (Eddington Luminosity) is calculated with this formula:

$$L_{Ed} \approx 3 * 10^4 * L_{sun} * \frac{M}{M_{sun}}$$

Example: For a black hole with $10^4 M_{sun}$, we get
$$L_{Ed} = 1.16 * 10^{35} W$$

My question is what does it really show? What do we get by calculating this Number? Is this formula really used to calculate the luminosity or it is used for calculating mass?


texts: What are the main differences between solar flares and coronal mass ejections?

texts: Specifically, what causes them, what sort of damage can they cause, and what are the differences in their composition?


texts: Why do rocks on other solar system bodies that have an atmosphere seem to be flat?

texts:
Images taken by landers on Titan and Venus and Mars show landscapes where rocks, to me at least, are surprisingly flat. Being used to walking around in forests with roundish meter sized boulders, I'd be very surprised to find myself in a landscape of flat rocks.

Their atmospheric density range between about 100 times, half and 1/100th of Earth's atmosphere. So that can't be the main explanation. Actually, I think that the landscape of the Moon is more Earth like than that of the atmospheric worlds out there.

Is this my impression of "flatness" a real phenomenon, and if so, why and how?


texts:
With every improvement of measurement we are able to determine the age of the Universe more exactly. I just wondered how much and what means it would need to narrow the range of age so that it would be possible to determine the exact date of the Big Bang.

I know that this would take a lot of assumptions for the sake of simplicity - I am just curious; this being said: let us assume that all current theories hold and no new side effects or new theories would evolve.

I read this post and some physics articles about the matter, but my question focuses on the accuracy gap between our current best telescopes and the accuracy that would be needed to narrow down the age to "24 earth hours".


texts:
I was going through all the questions in this community related to TRAPPIST-1 in order to know how planets TRAPPIST-1b to TRAPPIST-1h were discovered, but there aren't any.

How were they discovered?


texts: What if there was an asteroid belt with the mass of a planet?

texts: In the past, the asteroid belt was thought to be the remains of a shattered planet. Of course, that was all pseudoscience, the mass of the asteroid belt being 4% of the moon. I've been wondering, what would happen if this did occur. What would an asteroid belt with around 50% of the Earth's mass be like? How often would asteroids hit earth? Would it influence Mars?


texts: Was the use of silver on 'optical' telescope mirrors more common in the past? Why?

texts:
In the past, weren't many astronomical telescope mirrors periodically removed and 'resilvered', and wasn't this actually done using silver?

I'm guessing that this is much less common now - new telescopes are so large and complex that routine invasive procedures like that needed for maintenance of a silver coating would not be acceptable, and mirrors in many if not most older telescopes would have been 'updated' to aluminum.

Was silver used in the past because aluminum coating was too technically difficult, or because silver has slightly better reflectivity in the red, or some other reason?

note: I'm only asking about 'optical' telescopes meaning NIR & Vis, not the longer wavelength IR telescopes where IR emissivity is a concern.


texts: Could the planets in the Solar System have been captured?

texts:
Ignoring any current beliefs, is it possible that the planets could have been captured by the Sun's gravitational force after drifting through space while retaining their axis of spin and speed?

Notably, Venus uniquely has retrograde rotation; how would this have come about based on current theories?


texts:
This seems like a pretty basic question, but I couldn't find a duplicate upon quick inspection. From what I understand about black holes they actually contain no matter. All of the mass that maintains the black hole (ie. keeps space warped) is found in the energy bound in the space itself. I may not be stating this quite right, but this is basically what Kip Thorne states in "The Science of Interstellar". Therefore, it seems that to maintain angular momentum during black hole formation that space itself must "spin". This would imply that space is actually "spinning" even in the vicinity of the Earth (to a much lesser degree of course). Is this true?

As an aside, I don't take everything that Kip says as gospel truth. He states several times that Io is the innermost moon of Saturn in Chapter 7.


texts:
I’m working on a research project wherein we are trying to solve a problem very similar to VLBI delay. We have two radio receivers, and we know their locations. We also know at what time one receiver detected a signal, and the coordinates of it’s beam-pointing center. We want to determine at what time the second receiver should’ve seen the signal.
I understand in principle how VLBI delay is calculated; I’ve seen a few different formulas, such as the dot product of a vector between the receivers, and between the receiver and the source but practically speaking, how would I actually go about producing numbers?


texts: Creating a 2D matrix from 2 FITS images of unequal dimensions for Radio/FIR correlation

texts:
I have two images of NGC 6946:

item       description        size (pixels)    scale (arcseconds/pixel)
 1.     70 micron MIPS FITS     241 x 241           4.5
 2.     20 cm VLA FITS          512 x 512           2.0

I need to create a matrix with 2 columns (70 micron vs 20 cm), where each column will store the values at each pixel in such a way that the central pixel coordinate of each image coincides, so that I can plot for Radio/FIR correlation.

How should I perform such task in python?


texts: It sounds like dark matter makes up one quarter of the universe's mass-energy.  And it's density is fairly uniform.  But could it be simultaneously both uniform and moving - like currents in a lake.  Does it have a standing wave?  A propagating wave?  A rotation around some universal central point?  Local rotations around massive objects like low pressure systems on a weather map?


texts: Why are main sequence stars more massive than the Sun less dense? e.g. Vega, Spica etc

texts: Why do main sequence stars more massive than the Sun have lower densities? e.g. Vega, Spica etc.


texts: When Halley's Comet is visible in the sky, how long does that last? Is it only visible for a week or is it up there for a month, or several months? What I'm actually wondering is, though it arrived in 1910, could it still have been visible in the sky into 1911?


texts:
Some Background?
Coronal Mass Ejections, or CMEs for short are the topic of this question. Anyone new to this topic is sure to find some really interesting facts (and consequences) of CMEs and Solar Flares here.
Some Wiki definitions?

A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the solar corona. They often follow solar flares and are normally present during a solar prominence eruption. The plasma is released into the solar wind, and can be observed in coronagraph imagery.

So, what's the question exactly?
If you've seen the video linked above, then you know that CMEs release a lot of solar ejecta (plasma). The Earth's Magnetic field takes care of them in a way that we barely knew things like these even existed before some really advanced equipment came along.
So, Question - What effects do CMEs have on planets like Mercury?
I had trouble finding and/or verifying any resource that I found on this question. Can anybody help?
P.S. Any help with the tags will also be appreciated.


texts: What would happen if we put a small part of White Dwarf or Neutron Star on Earth?

texts: I would know what would happen if we put a small part (let me assume 1 cm$^3$) of White Dwarf or Neutron Star on Earth. Will it explode or maybe it collapses to the Earth's core?


texts:
I am having some issues with understanding what my adviser means by "galactic roll," but I have not been able to find anything online regarding this.

My project involves generating an array of az/el coordinates, converting them to ra/dec, then converting them to galactic lon/lat. I then use these coordinates to scan a Healpy map (all in Python).

She did mention starting off by setting horizontal roll = 0, and then converting horizontal coordinates to galactic coordinates. The galactic roll should then take on nonzero values, which is important to the data I am analyzing.

I've read about Euler angles and quaternions and I think I'm on the right path. She provided some code for me to reference, and that uses quaternions and Euler angles. Does galactic roll refer to one of the Euler angles (perhaps $\theta$)? Another reference code she provided mentions parallactic angle; could that be it? Any help or reference material would be appreciated. Thank you!

(And before you mention this, yes I will ask her. I just want to give this an honest attempt first.)


texts: What visible star is closest to the ecliptic?

texts: If we were to project the ecliptic out as an infinite plane, what visible star (besides the sun) would lie closest to it?


texts: SDSS - getting separate u, g, r, i, z frames from SWARP tool and astropy

texts:
I am following this paper to extract the u,g,r,i,z image frames from the SDSS image fits files. I have the fits files downloaded (using the bulk data download option from SDSS) and tried to run the SWARP tool to get the resampled image as output, which I will access through astropy. I am still unclear on the exact parameters that will make up the entire SWARP command. So far I have:
swarp frame-i.fits[0] frame-r.fits[0] frame-g.fits[0] frame-u.fits[0] frame-z.fits[0] -SUBTRACT_BACK N -RESAMPLE Y -RESAMPLING_TYPE LANCZOS3 -IMAGE_SIZE 64,64 -IMAGEOUT_NAME attempt.fits

This is written with the description of it in the paper, which is:

With the command that has been run, I am getting quite noisy images as output that do not relay any sort of information. What am I doing wrong in this process?


texts:
When a comet loses all his volatile material after many orbits around the Sun, it becomes a "dead" comet (like an asteroid, a centaur or whatever), see for example 2015 TB145.
Is there any way to determine the lifetime of a comet (expressed as the number of orbits before it becomes a dead comet)?


texts: 360 degree equirectangular projection of all naked eye stars with precessional motion for simulating

texts:
For many years now I've been using Voyager sky simulator by Carinasoft. The projection selections are great, but Equirectangular doesn't seem to one one of them. Is there a program out there that does this? I cooked up an Excel sheet to do the coordinate conversions, and I can import these into Autocad if I have to, but it seems like there should be an off-the-shelf package to do this.

For my current project I need to be able to show a 360 degree overview of changes due to precession from 3000-2000-1000 BC at a glance.  It may be my familiarity with the typical simple projection published for surveyors in a standard Ephemeris before the internet and GPS all but eliminated the demand for such publications.


texts: Does the Earth orbiting around the Earth-Moon barycenter cause a measurable centrifugal force?

texts:
I just realized the Earth is not a stationary object with the moon orbiting around it.  As shown in this minimalist animation from Wikipedia, the Earth actually orbits a common barycenter with the moon.  If this was a carnival ride, people sitting in the moon and on the far side of the earth would certainly feel the centrifugal force acting on them.

However, this is a very large system in reality, and the Earth also has quite a strong gravitational force felt on the surface anyway.  So when it comes to accurately measuring gravity or doing experiments that rely on gravity, how big of a factor is this centrifugal force?  Does the apearant force of gravity fluctuate from 9.9 m/s^2 to 9.7 or is it more along the lines of 9.800001 to 9.799999 (assuming the average is exactly 9.8, which is a simplification).  Or is there something I'm missing that means the force is non-existent?


texts: Internal heat and planetary differentiation

texts:
I was reading an article on planetary differentiation, and apparently internal heat production plays a major role. There are several sources of such heat described, such as tidal heat, radiogenic heat, etc. What I did not understand however is how, on chemical level, does warming up a system "heterogenize" it. Logically, wouldn't an addition of kinetic energy mean that molecules bump around more violently, therefore becoming more mixed up?

Or does this 'internal heat' story mean that the heat is actually escaping the planet, therefore kinetic energy is lost, therefore the planet is actually cooling down?

source: http://geology.isu.edu/wapi/Geo_Pgt/Mod03_PlanetaryEvo/mod3_pt1.htm


texts: Planet-timer "device" from 1970s or 1980s possibly by Edmund Scientific

texts:
Many years ago (early 80s or late 70s), I bought a cardboard "device" which I think was called "Planet Timer" and I think was sold by Edmund Scientific. Does anyone have any information on this device? Google was unhelpful.

The device had concentric revolving circles around a fixed Sun at the center. By revolving the circles to the current time (via tables provided with the device), you could presumably see what constellation each planet was in. You could also use it to approximate planetary conjunctions.

The circles were equal distance from each other, so I'm pretty sure the device couldn't actually have worked, but I'm curious to see if I can find out more about it.

Note: this is not a "planisphere" device, which did work, and for which I can still find plenty of information.


texts: Today I heard on the radio that the Super Collider has discovered mini black holes. They said that this discovery is proof of the existence of Parallel Universes and that the "Big Bang" never happened.I don't understand this and why this discovery means that Parallel Universe exist ? Does anyone know ?


texts: How can clouds form in Jupiter's atmosphere of Hydrogen and Helium?

texts:
Here is a graphic of cloud layers of Jupiter (source: Wikipedia):

There are three distinct cloud layers of ammonia, ammonium hydrosulfide, and water. The temperature and pressure conditions seem to be surprisingly earth-like; temperatures between 200 and 300 K, pressures about 1 to 10 atm, gravity around 1.3g.
Clouds (of water) form on earth because they solar energy causes them to evaporate from a solid surface, rise a few km, then condense to form water droplets (or solid crystal snow). But Jupiter does not have a solid surface, or nearly as much solar energy as the Earth.
All three of the cloud forming compounds should be liquid at the conditions of their cloud layer. Given the density of those liquids (between 0.7 and 1.2 g/cm$^3$) and the density of the bulk of the hydrogen and helium atmosphere, how do the clouds not fall as precipitation into Jupiter's interior and never re-emerge?


texts: Querying GAIA DR2 for Polaris (for example using https://gea.esac.esa.int/archive/) yields empty parallax columns. Anyone knows why it was not measured/published?


texts: How often are there five planet apparently aligned within 25°?

texts:
I chanced upon this in Stellarium.

Is it common for five planet appear to align within 25°-30°, viewed from any of the inner planets?


texts: What supernova has created the iron currently found in Earth core?

texts:
Iron is generated by stars in a certain part of their life cycle. Earth contains a lot of iron inside, however it is clear that this iron could have not been generated in a star in close proximity.
If we consider the current Earth position as reference, where would have lied 4.5 billions years ago the stars that generated the iron currently found in Earth core? Can we say that in the current Earth position there was a huge star 4.5 billions years ago? How did the iron formed in these old stars reach the primordial gas cloud that turned into the solar system? For how long did it travel? Can we say that it came from a set of stars that lived (again, 4.5 billions years ago) in the area that we now call the galaxy core, for example? Can we define the set of "iron generating" areas? Can we define something like the "Earth core iron generator sphere"?


texts: By convention, when observing an exoplanet, is the observer's line of sight (on Earth) perpendicular to the plane of sky?

texts: This is just a quick question. In this picture of the orbit of an exoplanet, is the observer's line of sight perpendicular to the plane of reference, which is in the +z direction on the diagram? In the case when the inclination equals to $90^{\circ}$, it is ideal for us to measure the radial velocity. Is that right?


texts: What effect does a natural satellite have on a planet's rotation and revolution?

texts:
how is the rotation and revolution of a planet that does not have a
 natural satellite different from  a planet that has say 1 natural satellite
(ex. earth and moon system).


texts: Are we moving away or towards each other?

texts:
There are numerous theories about the shape of the Universe. I've read somewhere most probably in 'A Brief History of Time' that there is a possibility that the Universe is spherical. So is it possible that all galaxies are moving towards each other? Imagine the Universe as a sphere. Now, imagine a diameter AB. The Big Bang took place at A and the galaxies started drifting away and as they moved away further they stared attracting each other on the other side i.e.Gravitational attraction. So we feel that the farther galaxies are the faster they are moving away(Hubble's Law) but actually they are moving towards each other and will eventually meet at point B. Also, space-time is spherical and so light travels along the sphere.

So, my question is: Is it possible for the universe to be spherical with galaxies apparently moving away but actually moving closer?


texts: Are metallicities of molecular clouds lower in the outskirts of the galaxy?

texts:
(this question was originally posted in an answer by user PSR-1937-21 to another post. I find it an interesting one, but since they don't seem to be active anymore, I'm posting it to see if somebody knows the answer.)

From how mass is usually distributed, it seems reasonable that metallicity would be lower in the outskirts of the galaxy. If the metallicity is lower, would that allow for larger stars to form? (see for instance "Why is metallicity important in the death of stars?"). Maybe there is a larger Jeans mass due to warmer clouds, the result of less efficient radiation of thermal energy?


texts: Why was Dawn's (the spacecraft) flight path circular?

texts: In comparison to the twin Voyagers, Dawn's flight path circled the sun in an anti-clockwise manner. The map of its path shows its targets, Ceres and Vesta were in a trajectory that was close to a straight path, if Dawn had taken a clockwise direction, with a close flyby of Mars. I can't find a reason to show it was studying the space around Lagrange point 3 (in relation to Earth), as according to this flight path map. So what was the reason for a longer direction?


texts: How could a hobbyist astronomer determine apparent magnitude of a star?

texts:
Apparent magnitude is a rather complex way to determine the brightness of a star. Quoting the introduction text from the linked to Wikipedia page:

  The apparent magnitude (m) of a celestial body is a measure of its
  brightness as seen by an observer on Earth, adjusted to the value it
  would have in the absence of the atmosphere. The brighter the object
  appears, the lower the value of its magnitude. Generally the visible
  spectrum (vmag) is used as a basis for the apparent magnitude, but
  other regions of the spectrum, such as the near-infrared J-band, are
  also used. In the visible spectrum Sirius is the brightest star in the
  night sky, whereas in the near-infrared J-band, Betelgeuse is the
  brightest.

While it is of course a useful measure also for the slightly more casual, non-scientific observer to help determine the observed star from its neighboring cluster, or identification in general, I always wondered if there is a way to measure apparent magnitude with enthusiast-class equipment, and what would these procedures be?

                              Apparent magnitude scale and observational limits (Source: ESA Science)

Also interesting would be a description of what level of precision and enthusiast can get with such equipment while measuring apparent magnitude of a distant star. If you need specifics to answer the question, such as precise available equipment or subject of observations, please feel free to choose at will any that would broadly match the capabilities of enthusiast-grade equipment.


texts: In a finite universe, what happens when light reaches the boundary?

texts: If a universe is finite and is not expanding at a speed equal to or greater than c, what happens when light or another form of electromagnetic radiation traveling reaches the boundary?


texts: Can we violate law of physics in any other planet?

texts: Can't there be any way to violates any law of physics? If in our earth we can't violate any law of physics can't we violate in any other planet?


texts: What is the Schwarzschild radius actually?

texts:
I am currently writing a detailed essay about black holes, the history of their discovery and their characteristics and I can't find the appropriate literature online and in my local libraries (I even looked in the uni libraries specialized for astrophysics, the books are just way too old) which explain in more detail what the Schwarzschild radius is.

I'd like to understand it more rather than copying those short wiki explanations and accepting the calculations just the way they are.

I'd really appreciate the explanation from the core of  the definition of the Schwarzschild radius in order to understand the rest of the complex mechanism of black holes.


texts: Is there enough hydrogen left after a star dies so another star will have enough to light up?

texts: A star consumes quite a lot of hydrogen in its life, and is pretty much "vacuuming" everything in its vicinity. After it dies (eventually by supernova which will spread all its composition over light years), is there enough hydrogen left in that area to light up a new star? And will that star be more short-lived compare to its predecessor?


texts: Does coronal mass ejection include radiation at optical wavelengths?

texts:
It is known that during Solar/Stellar Flare, plasma from the corona of the star is released into the space, and travels comparatively slowly.

In an hypothetical situation, does the coronal mass also consist of electromagnetic radiation within the optical wavelengths? And if so, is it possible to observe the incoming coronal mass ejection from the ground/surface of the Earth way before it approaches?


texts: total energy and apparent bolometric magnitude

texts:
So I have been given this question:

  A typical Type Ia Supernova emits $10^{44}\ \mathrm{J}$. All nuclear weapons
  tested by humans have released a total of about $2\times10^{6}\ \mathrm{TJ}$ of energy. Assume the Supernova energy is released at the distance of the Sun. Determine how close to your retina the nuclear weapons energy would have to be released to make both appear to have the same apparent bolometric magnitude.

The issue I'm have is normally apparent magnitudes I would have to have joules per second where this is in just total joules.

So is it that with bolometric magnitude there is an automatic assumption that when total power is mention you assume joules a second?

Could someone please advise?

I would just like to point out I am not looking for a direct answer to this question, I am just confused with why total energy has been used. I have read posts on wiki which seems to confuse me even more. I have just used the above question as an example from a book.


texts:
I'm more in the CS based stack exchange, but this question prompted me to come in here.

I've been compiling a large set of ephemerides for solar system, stars with mag > 6.8, exo planets etc, and I am now working on the DSO catalog.

The app actually holds celestial objects in 3D space, and I made conversions from RA/DEC/dist(parsec) for various objects, but I am having trouble with DSO objects.

I know of a red shift time base formula, but heard the conversion is not very good. Also, I understand that most estimations cannot be counted as precise, but I would like to have these objects within 3D space.

Can anyone tell me what features I should look for in a catalog and what formula I can base myself on for this conversion, if any?

thanks


texts: Is there a collection or list of all planned observations of 99942 Apophis during its 2021 close pass to Earth?

texts:
For example, I found Goldstone Radar Observations Planning: 99942 Apophis in 2021 and 99942 Apophis 2021; Planned, Scheduled and Completed Observations at lawn.com? which seems empty.
But I'll bet the observation campaign for this important event by various means (radar, optical, infrared...) was well coordinated. Because of the potential long-term risk of impact with Earth it's necessary to learn as much as possible about 99942 Apophis.


texts: I understand that as the universe continues to expand, eventually our supply of hydrogen will be exhausted and then it's "lights out."  Is that really going to happen or is there something "eternal" about the universe?  It matters because if the universe is eternal, and since we are a part of the universe, we too are eternal as well in some sense.


texts: Is the Hubble Ultra-Deep Field image representative of the entire sky?

texts:
The Hubble ultra-deep field image is a photograph of an area of sky equivalent to a 1mm x 1mm piece of paper held a metre away from the eye (one thirteen millionth of the entire sky). It contains an estimated 10,000 galaxies.

If galaxies were uniformly distributed, and the HUDF were representative of the entire sky, that would mean there were approximately 130 billion galaxies, which falls between estimates I have read of between 100 and 200 billion as the total number of galaxies.

However, my understanding is that galaxies occur in clusters, which would seem to imply that some areas of the sky should be more densely populated than others.

So is the HUDF representative of the entire sky, or are some areas much more and much less densely populated?


texts: How to find maximum and minimum right acsension and declination based on the telescope's location?

texts:
If I am to apply at a telescope for viewing objects, how to determine (easily) what is the minimum and maximum possible RA and DEC visible from that telescope during that time of the year. For eg, take the telescope at paranal
(24.6258°S, 70.4310°W) and if I observe anytime from October to March.


texts: How long does it take the center of the Earth to complete a full orbit?

texts:
If we take midnight of 1/1/2017 in any country and mark in space where the center of the Earth is located, at what date, in the same country, has the center completed a full orbit.

I am almost certain that has to be 1/1/2018, but I am not so sure about the hour.


texts: Is an elliptical orbit around a black hole, or an orbit consisting of three bodies (including the black hole) possible?

texts: If this is possible, would it then lead to an object undergoing varying time dilation (predictable variation with elliptical and random variation with three body)?


texts: Would dropping a white dwarf or a "still-hot" stellar core into a large planet or brown dwarf create a star?

texts:
In the Wikipedia article about stellar engineering, I found this quote:

In The Saga of the Seven Suns, by Kevin J. Anderson, humans are able to convert gas giant planets into stars through the use of a "Klikiss Torch". This device creates a wormhole between two points in space, allowing a neutron star to be dropped into the planet and ignite stellar nuclear fusion.

Now this is definitely impossible, as neutron stars are degenerate and cannot fuse. But what if we dropped a helium white dwarf or extract the hydrogen-burning core of a low-mass star and put it into a large gas giant or non-fusing brown dwarf? Would this trigger fusion or would this fail as well (presumably because the white dwarf is also degenerate)?


texts: Where do we have it from that the Moon is migrating away from Earth?

texts:
I know Neil Armstrong placed a mirror on the Moon and people shoot lasers there which get reflected, by this, measuring the time the light is needing, they can conclude the current distance of the Moon. But the Moon is once in a time moving away from the Earth (when annular eclipses are possible) and once in a time getting closer (when total eclipses become possible). Sometimes we hear of records like "the Moon is as close as it wasn't for decades". I remember about one year ago I saw a very close and bright full moon, almost outshining most stars.
So how is it concluded that the Moon would constantly move away from the Earth? Is it just based on the physical prediction of celestial mechanics, like calculation that a day on Earth would get longer and the Moon's orbit slower?


texts: In km/h, what actually is the "speed" of Andromeda away from us: cosmologically?

texts:
Andromeda is about 2.5 million ly away.

Actually, in this universe, at what "speed" (in km/h) are two objects separating cosmologically - I mean strictly due to the "expansion of the universe" - if they are 2.5 million ly apart?

I do understand that local "ordinary" or "peculiar" motion completely swamps this effect.  If I'm not mistaken, the "local" "ordinary" motion of Andromeda per our galaxy happens to be about 400,000 km/h towards us.

Is the "speed" due to the "expansion of the universe" drastically smaller than this?

I assumed that the expansion of the universe (or "of the spacetime metric") is even everywhere: it's well known that it only affects "the largest structures" but I still assumed that the expansion is the same in my room, my galaxy, my cosmological region.  Perhaps this assumption is totally wrong?


texts: Viewing a solar eclipse through a leafy tree

texts:
Viewing my Facebook feed today, my local news station posted regarding a solar eclipse taking place today:

Note the line about using a leafy tree as a filter:

  Scientists say NOT to directly look at the sun (if skies are clearing where you are), but looking at the eclipse through a leafy tree creates a natural filter!

I've heard a lot of people recommend viewing a solar eclipse by looking at the projection of the eclipse caused by light passing between leaves in a tree, in this case looking at the ground or a wall, where ever the light hits after passing through the tree.  However, the Facebook posting here seems to recommend looking up at the solar eclipse through the tree.

I suspect that they have misunderstood the projection method (or inadequately explained), and their advice is not only wrong, but potentially dangerous.

Is this method recommendable? Should it be considered dangerous?

Follow-up

Almost immediately upon having seen the posting I had commented on it noting my belief that such advice could be dangerous, including a more clear explanation of how viewing using trees should be done. The staff member who originally posted the status later took note of this and edited the status to less ambiguously explain viewing the projection. Unfortunately, this was done after the solar eclipse had concluded. Hopefully no damage was done.


texts: What can we expect to see with a telescope with a 70mm aperture and a 10mm eyepiece?

texts: We purchased my young daughter a beginner telescope for Christmas and she's super excited to look at the large planets. Our telescope has a 70mm aperture and two eye pieces (10mm and 20mm) How much, if any, detail can we expect to see on Jupiter and Saturn? I don't want her going in expecting to see Saturn's rings and come out disappointed! We are very new to this and I want her to have the best experience possible with the equipment we have. Thanks!


texts: Are there any known spectral lines shifted by ~1100? If not, then how certain is mainstream that the CMB has a redshift of ~1100? All I see is a blackbody radiation curve void of spectral lines.


texts: A plot of the Moon-rise/sets on the Horizon Location

texts:
We have been trying to see where the moon rises/sets on the horizon.   Unfortunately, the moon is not cooperating.   Even when we know the moon rise/set times (timeanddate.com), we often cannot find the moon on the horizon.
We are wondering if the moon wobbles back and forth along the horizon the same way the sun wobbles.  We cannot find any website that talks about where the moon is on the horizon.


texts: Can a black hole which has an enormous mass and spins so fast cause spacetime to to be stretched to is absolute limits that it  start so rip?


texts: How did they decide the length of one year even though it wasn't known that earth revolves around the sun?

texts: It takes one year for the earth to go around the sun, but one year was already this long when they didn't even know that the earth revolves around the sun at all. So how did they figure it out?


texts: Is there a theoretical maximum size limit for a star?

texts:
Some stars are simply huge. Eventually, though, wouldn't there be simply too much pressure or mass for the star to sustain itself? Wouldn't it eventually collapse into a black hole?

Is there a theoretical upper limit on the size of a star, and what is it based on?


texts: What makes small interferometers useful? Like NIRISS on JWST

texts:
NIRISS is an instrument on the James Webb Space Telescope. It has a "non-redundant aperture mask" which obviously covers most of the area of the sensor. It seems to be advantageous for high contrast imaging (like finding an exoplanet next to a star) and an alternative to coronagraphs. But however does that work? Why is it good to cover most of a sensor?

I have associated interferometers with creating as large as possible baselines for higher resolution, like the Very Large Baseline Array and the Spectr-R radio space telescope which gives up to a 390,000 km long baseline. So what is the magic with sacrificing sensor area to turn a single small telescope into an interferometer? Aren't all photons welcome? Would such an instrument do as well with a correspondingly smaller main mirror (maybe in separate fragments)?


texts: What are the biggest problems about the numerical, finite-element GR models?

texts:
As I know, for example the modelling of the collapse of a neutron star (to a black hole) wasn't done correctly until now.

Why? Yes, I know, the Einstein Field Equations aren't really easy to solve. But, for example, the situation is very similar in the case of the QCD, but there are already fruitful results existing ( http://physicsworld.com/cws/article/news/2008/nov/21/proton-and-neutron-masses-calculated-from-first-principles ).

Is there any theoretical obstacle in the way? Or we simply didn't have enough fast computers/physicist/programmer manyear until now?


texts: How does the amount of dark matter in measured galaxies vary?

texts: I have recently viewed two news articles announcing a galaxy composed almost entirely of dark matter and one with hardly any. This made me wonder if there is a "continuum" of percentages of dark matter in galaxies, where it has been measured, or if there is a "preferred" range.


texts: Which galaxy is closest to the center of the KBC void?

texts:
I was reading up on Bootes void and came across this list of the largest voids in the visible universe, and apparently the one we're in, the KBC void, is not only the biggest, but damned-near spherical.  The Wikipedia page says that the Milky Way is within a few hundred million light-years of the center of the KBC void, but I'm unable to find out what, if anything, is closest to the center.

Is this information available anywhere?  Does anyone know what's at the center of the KBC void?


texts:
I am trying to find a source of mirror reflectivity versus wavelength for the typical protected overcoat Aluminum mirrors used in astronomy. I need this to cover the whole visible range (300-1200nm) and ideally machine-readable. I've only been able to find part coverage of the wavelength range or single spot measurements at a specific wavelength.

The best I've managed is this page at Edmund Optics which has (small) plots but no table or downloadable file.


texts: If oxygen was abundant in Neptune, would there be combustion?

texts:
Since Neptune/Uranus have high percentages of methane, wouldn't it be highly likely that there will be combustion (triggered by the lightning storms or any other factor) if oxygen was abundant?

Will the hydrogen combust as well?


texts: What is the difference between aurorae and electroglow?

texts: One of the discoveries of Voyager 2 at Uranus was a phenomenon called "electroglow", which as I understand it is related to charged particles interacting with the atmosphere that cause the emission of ultraviolet radiation. What I am not clear on is how this process is different to the production of aurorae. Is there some clear difference between the two processes that allows them to be distinguished?


texts: Where do stellar classes names come from?

texts: Under Morgan-Keenan classification stellar classes are called O, B, A, F, G, K, M, but what is the logic behind these letters? I can't see any pattern and I doubt they have just been chosen randomly. I also wasn't able to find an answer on the Internet, so is there any reason stellar classes are called this way?


texts:
Given its right ascension and declination, find out the constellation a celestial body is in.
Does anyone know of an algorithm (C#, js) for that?
It would read the constellation boundaries (e.g. constellations.bounds.json, at https://github.com/ofrohn/d3-celestial/tree/master/data) and figure the constellation out.
Thanks in advance!


texts:
Carbon is considered volatile by planetary science, eg Moon lacks volatiles and thus lacks carbon.
However volatiles are defined as "elements or substances with low boiling point", but Carbon boiling point is very high! Its sublimation point is 3900K, so it should be refractory and not volatile.
I am confused why it is volatile with such high sublimation point.
Regards,
Alex


texts: A simple echelle spectrograph for viewing solar spectrum

texts:
On the Astrosurf website here, someone has designed a deceptively simple echelle spectrograph for viewing solar spectrum. Briefly, light from an optical fiber falls on an echelle grating and which is cross dispersed with a prism.

In theory, an echellogram has a range of wavelengths (free spectral range) in various orders as follows:

I tried to make a crude version of this (without an optical fiber or a collimating lens) for educational purposes.

With a white LED (from an iphone or simple LED flashlight, or a LED bulb) shining on the echelle at a proper angle, one can generate a pattern as follows, which looks impressive and looks like a true echellogram. Unfortunately, this success is rather short lived. The moment we use a fluorescent lamp with a slit infront, we just get repeating images, which shows that this is not an proper echellogram! Does anyone know why this is happening? A senior spectroscopist has also tried it and no matter what orientation of slit or blaze direction (against or towards), grating angle, grating orientation... the result is just a repeating image.
Questions:
a) Is it just because there is no collimator in our crude version? Why is an LED working and line source not? I tried collimation with a lens, but there was no improvement (repeating lines were still there).
b) If this were a proper echellogram of a line source, such as a fluorescent bulb, one should see one purple line in first row (higher orders), two green lines, two yellow lines (Hg doublets) and a few red lines. I cannot understand why there is a repetition of the same wavelength say purple line or the two green lines?
Thanks.


texts: What is the significance of using baseline pairs in radio interferometry?

texts: Radio interferometry utilizes arrays of smaller telescopes that are linked together to synthesize a larger aperture telescope. Astronomical radio observatories, such as the Very Large Array in New Mexico, consist of multiple radio antennas in various configurations (ie. 27 antennas for the VLA). When I read about the basics of radio interferometry, a lot of time is spent on the delay and baseline between a pair of telescopes. In other words, the light from the object will enter one telescope first before the other (unless pointed directly up) and a pair of telescopes act as 1 baseline. I don't understand the significance of these telescope pairs. Do the pairs act as a single pixel? I read that a pair of antennas measure one point in the u,v plane. Why are pairs of telescopes needed to do this and what is the significance to radio astronomy?


texts:
I'm getting into scripting in Stellarium, and it's fairly similar to the scripting language used in a real Planetarium I used to work in, which used DigitalSky2, if memory serves. In the Planetarium I was able to write code "in real time" as such - meaning I could write one line and run it, like writing Python code into Terminal, and in doing so I was able to learn a lot about how it worked (and break it - all. the. time.)

This having to write a script, upload it and run the whole thing is less than efficient, and doesn't suit my learning-a-new-language style. Especially when I'm really just wanting to play around with the script, one line at a time, and see how I can best break it... I mean, tests its limitations!

Is there any way of doing this? Or is it a pipe dream I have to let go of for the time being?

Cheers!


texts: How can professional astronomers keep amateurs engaged given the ever-increasing power of modern telescopes?

texts:
There're many amateur astronomers out there who build their own telescopes, write their own data-gathering programs, and scan the night sky tirelessly. Sometimes they even happen upon new discoveries simply because they had their telescopes pointed in the right direction at the right time.

However, the telescopes used by professional astronomers get more and more sophisticated all the time. This means that the window for amateur astronomers to contribute gets smaller and smaller all the time. For example, the recently-discovered Comet Borisov was discovered by an amateur, but the number of comets discovered by amateurs has been constantly decreasing. Which is obviously rather depressing. One day someone might write a poignant documentary which goes "I thought I was contributing, but then the professionals built [this telescope] and I've become redundant ..."

These citizen astronomers are obviously great PR for astronomy, but given the above, how can professional astronomers keep them engaged?


texts: Has the Earth-facing side of the Moon that we see today always faced us ever since the Moon got tidally locked? Or does it precess?

texts:
Title. Does the Earth-facing side of the Moon slowly precess due to perturbations and torques exerted by other bodies? Or is the side of the Moon we see today the same as when it first got tidally locked?
P.S. I know we don't see the exact same side of the Moon   everytime due to libration. That's not what I'm asking.


texts:
I'm asking this question as a follow-up to if light has no mass why is it affected by gravity?

Imagine you’re standing on a gedanken planet, shining a laser beam straight up into space. The light goes straight up. It doesn’t curve, and it doesn’t fall back down. Now imagine it’s a denser more massive planet. The light still goes straight up. It still doesn’t curve, and it still doesn’t fall back down. Let’s make it a really massive planet. That light still goes straight up. It still doesn’t curve, and it still doesn’t fall back down:

But when we make our gedanken planet so massive that it’s a black hole, all of a sudden light can’t escape. Why? Why doesn’t the light get out? Why doesn't the vertical light beam get out of a black hole?


texts: Why are there so many seemingly blue-shifted galaxies in deep space

texts: NASA released this image of deep space. The way light shifting vis-a-vis universe expansion is supposed to work is by making the furthest galaxies appear mostly red-shifted. But NASA's image shows somewhat of an even balance between blue and red shift. Why is that?


texts: Why do some electromagnetic waves continue travelling while others disappear?

texts: Cosmic background radiation emitted when the Universe was very young still exists. But my wifi signal seems to disappear a short distance from my apartment. Why?


texts: I was looking at the Wikipedia article "List of potentially habitable exoplanets" and I noticed that many of the closest planets listed(tens or hundreds instead of thousands of light-years away) orbited M-class stars. I have also read that scientists are more certain about the possibility of habitable planets around G and K class stars, because the habitability of red dwarf systems is still debated. I was wondering whether there is something that makes the larger stars be farther away from each other, or whether it is just a coincidence. So is it possible to have many G and K class stars tens of light years near each other?


texts: How could the polarizer in early coronagraphs make the Sun's corona more visible against the daytime sky?

texts:
Wikipedia's Coronagraph; Invention says:

  High Altitude Observatory's Mark IV Coronagraph on top of Mauna Loa, uses polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle and therefore undergoes scattering polarization, while the superimposed light from the sky near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.

How does this work?

For example, I looked at the daytime sky through a polarizing filter and I didn't notice substantial sky darkening? How could a polarizer have darkened the sky sufficiently to make the solar corona more visible?


texts: Speculative Question Concerning the Planet Theia

texts:
I am a writer and I'm building a paradigm wherein the planet Theia did not strike the Earth, but settled into a orbit around the sun a third of the distance between the Earth and Mars. I wanted to ask people in the astronomy field how that might affect the climatic development here on Earth.
I have designed the paradigm where Theia is in a slower rotation around the sun causing periods of time when the two planets will be separated by the sun due to the Earth's faster orbit to being near each other in orbit as the Earth "laps" Theia.
I'm curious how the "waxing and waning" of Theia's proximity to Earth might affect our planet's climate, especially in the near periods. In the story, I assume the development of sentient life and consider how they would experience the possible climatic events when Theia is near. I am wanting to understand the possible range of effects this would have. Would they necessitate alterations in daily life? Would they be dangerous at all? Or would they be cataclysmic in nature?
Also, wondering if anyone would know of a good model to follow describing the actual time between periods of near proximity between Earth and Theia assuming a similar elliptical orbit.
To anyone who responds, I appreciate your time. I want to make sure I get this right or as close to right as possible. I just don't know the physics beyond what I imagine might happen. Thank you all.


texts: About the formula for taking in count of the observer altitude in sunrise/sunset formula

texts:
I'm new here and searched for solution but did not found one. I have found algorithms in order to calculate sunrise and sunset times for an observer on the surface of Earth with explanations of derivations of these formulae, but I want to improve it by including the altitude of the observer. I found an article in Wikipedia.

Could one explain to me, from where is this "2.076 degrees" in the formula $-2.076^{\mathrm{o}} \sqrt{\mathrm{elevation\:of\:observer\:in\:meters}}/60^{\mathrm{o}}$ derived and calculated?

Thanks in advance!


texts: Why is Io's volcanism chemically so different from Earth's?

texts:
Volcanic gases on Earth are mainly water, carbon dioxide and sulfur dioxide, but on Io there is a notable absence of water and carbon dioxide in volcanic gases, since sulfur dioxide is the dominant gas expelled by volcanoes in this satelite, why does this happen? It seems strange to me that water is not an important volcanic gas on Io when it's quite common in the composition of the other Jovian satellites.

In addition, the Io's surface is very rich in sulfur, which comes from volcanoes, but other bodies of the solar system that also experienced intense volcanic activity at some time have a surface dominated by basalt, such as Mercury or Mars. So, if Io has had intense volcanic activity for a long time, why doesn't it have a dark gray basalt surface like Mercury but a yellow one because of sulfur?


texts: Why doesn't the earth's surface collapse onto itself?

texts: From what I know the sun (or any other star) nuclear fission keeps it from collapsing under its own gravity until the fuel runs out, if that's so why doesn't the earth surface like the tectonic plates collapse to the earth's core? is it because of similar processes that happen in the earth's outer core (nuclear fission)? and if so what would happen if they would once stop?


texts: Instrumental magnitude to "real" magnitude - Photometry with not enough data?

texts:
I have a lot of data taken in R, B, V, Luminance and Halpha-bands which I want to analyse photometrically (one target in the frame).

The frames are already reduced (with flats, darks etc.) and have a correct wcs-header. I also already have the instrumental magnitude of my target and other stars in the frame (using APT).

My question now is, how do I calculate the real magnitude of my targets?

The most simple way would be comparing the other stars' instrumental magnitude and real magnitude and then applying the (logarithmic/linear?) transformation to the instrumental magnitude of my target. But I don't know how exact that would be and how I could calculate the error of the photometric solution (by applying the same transformation to the error of the instrumental magnitude APT gave me?).

For more exact photometry I don't have enough data, aka air mass or calibration fields.

Do you have any ideas or solutions?


texts: Time domain astronomy and fastest eclipsing binary ZTF J1539+5027 (+20 mag, 6.91 minutes): How to measure its minimum brightness?

texts:
Per Wikipedia's ZTF J153932.16+502738.8

ZTF J153932.16+502738.8 is a double binary white dwarf with an orbital period of just 6.91 minutes. [...] The light curve shows eclipses. One dip in the light curve is 15%, and the other is close to 100%.

Below is a light curve plotted on a linear scale in Fig. 1a of Burdge et al, (2019) General Relativistic Orbital Decay in a 7 Minute Orbital Period Eclipsing Binary System. The dip at 0 phase is deep.
A logarithmic magnitude scale might tend to show just how deep but noting that the minimum is dimmer than +21.5 mag and probably lasts only a tiny fraction of the orbital period (i.e. a few seconds) makes a good measurement of the minimum quite a challenge.
It seems to me like this might be a situation where a photomultiplier tube and photon counting might be competitive with photometry based on CCD imaging and clever readout schemes.
Questions:

What are the technical challenges to measuring the minimum brightness of this eclipsing pair?
If it could be measured, would such a result even be particularly useful in this case?

Figure 1: Lightcurve of ZTF J1539+5027 a) The binned CHIMERA g' lightcurve of ZTF
J1539+5027, phase-folded on the 6.91 minute orbital period. At phase 0, the lightcurve exhibits
a deep primary eclipse, indicating that the hot primary star is producing most of the observed
light. Outside of eclipse, there is a quasi-sinusoidal modulation because the primary star heavily
irradiates one side of its companion. At phases ±0.5, the secondary eclipse occurs as the hot
primary transits the irradiated face of its companion. b) The phase-folded ZTF g-band lightcurve
of the object. We were able to discover the object because of its periodic behavior. c) A binned g' lightcurve obtained with KPED, phase-folded on the orbital period. Error bars are 1σ intervals.


texts: Is it thought that the majority of Earth size or larger terrrestrial planets have retained most of their original atmospheres?

texts:
I used to think that most terrestrial planet atmospheres were secondary (from out gassing). However, after reading this https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140017487.pdf and noticing that the ratio of CO2 to N2 is far higher on Venus than that just expected from out gassing, I have done a 180.

I'm thinking that Earth has a primarily secondary atmosphere because of the Moon creating event and smaller planets like Mars just aren't massive enough to retain original nitrogen, etc.

Would this cast a shadow on the idea of early extreme stellar activity ripping off atmospheres of close in planets?


texts:
Outside of a specially crafts environment, could liquid water exist on Mars? Not some time in the distant past, but within the last few years. Mars' atmosphere is too thin for water to exist, right?

Perhaps if the water was mixed with salts or other minerals, its sublimation point might be altered enough to exist? Mars can get warm during its summer days, up to about 20 degrees Celsius at the equator. Perhaps, in certain areas and time periods, we may get a kind of liquid water?

Sorry for the speculative nature of this question, but after NASA talked about the possibility of liquid water, it got me wondering if it was possible.


texts: What does the coordinate of time really mean?

texts: The meaning of the $t$ which appears in space-time intervals or metrics in general relativity. I concluded that $t$ was just a mathematical thing which allows us to label the "space-time manifold" and only proper time τ had a physical meaning too. On Wikipedia I also found that the coordinate time is not a time that could be measured by a clock located at the place that nominally defines the reference frame. What does it really mean?


texts: What information will SETI have access to from the SKA (Square kilometer array)

texts:
I read on on the website of the SKA (https://www.skatelescope.org/) that SETI will sort of have "passive" access to the data that they capture on the array, sort of like a "pigyback" concept and search the data for signals.

How much of the data will they have access to? Everything? That would be literally petabytes of data per day that they would have to process. If so how do they plan to process all that data? Like a more effecient SETI@Home?


texts: How to take into account the filter response in thermal and reflected flux density calculations?

texts:
I'm using python to calculate the thermal and reflected flux densities of a faceted asteroid model, where both equations have the Planck function present in them.
Currently, I'm integrating the Planck function over the bandpass of the filter. For instance, using the Spitzer Space Telescope's IRAC channel 2 filter, I'm integrating it with an effective wavelength of 4.493 microns with a bandwidth of 1.010 microns. Is this correct way to do it, given this filter has a top-hat filter distribution?
For the case, of the reflected flux density, I'm integrating it with the bandpass of the Johnson standard V filter with an effective wavelength of 0.551 microns and a bandwidth of 88 microns. However, this V filter has Gaussian looking distribution. How do I take this into account?


texts:
Pulsars are neutron stars the emit a beam of electromagnetic radiation that is not aligned with their rotation axis. So we can only see a neutron star as a pulsar if the beam crosses Earth's path.

The common representation is that there are two beams, one on each side, but unless the magnetic axis and the rotation axis are at 90°, we can only ever see one of the two beams. How do we know that there are two beams and not one, or four, or any other number?


texts: Why has Moving Cluster Method been successful only for Hyades?

texts:
At all places where I have read of Moving Cluster Method for measuring distance to a cluster and once applied myself on data, I have been told that the method has been successfully applied only to Hyades cluster in Taurus.

Question : What factor made the method unsuccessful for other known clusters?


texts: Tradeoffs "upgrading" from a 60/900 refractor to a 150/1200 Dobsonian?

texts:
My current/previous telescope is a Skywatcher 60/900 refractor on an equatorial mount. I am now looking at a SkyWatcher 150/1200 "Classic" Dobsonian reflector.
How can I compare the two in terms of ease of use? What will be easier to do, and to see with the larger Dobsonian?  What will be harder?
I want to see planets, galaxy, nebula, etc.


texts: The contribution of components like dark matter, stars and gas to the mass of a galaxy

texts:
I am doing an introductory course in astrophysics and astronomy undergrad level.
In it we were taught that the main components of matter in a galaxy are dark matter halo ($M_h$), gas ($M_{gas}$) and stars ($M_*$). Thus the mass of the galaxy can be given as the sum of these three components. I want to read more about this but when I search this on net I don't get any appropriate results or they are very advanced. I guess this is because the actual terminology and nomenclature of these masses is different.
Thus if anyone can give me accurate terminology of these masses it would be helpful.
Additionally we were told that given the mass of dark matter halo the other two masses can have only a certain range of values. I want to know how we can determine these range of values possible for a galaxy.
Also how do these values change as the galaxy ages?
Any links to resources regarding the above topics are also welcome.
Edit 1: Regarding my question above about the change in mass of galaxies over time I am giving my thinking. Please correct me if I am wrong somewhere.
Initially galaxy consists of only dark matter and gas. This initially contracts but later undergoes segmentation to form stars. Thus initially there should be decrease in $M_{gas}$. But slowly some gas is returned as stars die and also gathered from surroundings, hence the gas mass should remain constant or decrease at a very slow rate. Now for the stars initially $M_*$ should increase due to creation of new stars. But after some time as some of the early stars start dying out there will be simultaneous birth and death of stars, hence the $M_*$ should fairly remain constant during this period. Finally as the star formation rate decreases due to depletion of $M_{gas}$ the birth of new stars will be reduced. Slowly more and more stars will start dying and $M_*$ will be depleted leaving behind only heavier elements. Hence $M_*$ should decrease during this period. Finally coming to halo mass, I am still not sure about it. I think it should not change much over the lifetime of the galaxy because dark matter does not seem to interact with the baryonic mass. I know that these changes will be slow but can anyone give me a rough number on the slowness of these changes?


texts: Why is the "green" comet C/2014 Q2 (Lovejoy) red in this picture?

texts:
In this nice NYTimes 'survey article' "Our Vast Solar System
and Its Many Explorers" there is a NEOWISE image of Comet C/2014 Q2 (Lovejoy). See here also.

The NASA description of the image says:

  The red color is caused by the strong signal in the NEOWISE 4.6-micron wavelength detector, owing to a combination of gas and dust in the comet's coma.

So I understand that this is a false color image, and the image at 4.6-micron is overlaid using red. So I should really ask: why is Comet C/2014 Q2 (Lovejoy) (and probably many) comet actually bright at 4.6 microns "...owing to a combination of gas and dust in the comet's coma." Is the gas fluorescing and the dust scattering? Or is this thermal radiation from the dust? The explanation in quotes doesn't really explain why "the red color is caused by the strong signal (around) 4.6-micron wavelength (is) owing to a combination of gas and dust in the comet's coma."

below: The more familiar green glow of this (and many) comet from here.


texts: Why is moon light not the same color as sunlight?

texts: The light from the moon is light being reflected from the sun. The sun, in space, is white. But on Earth, when the light is filtered through an atmosphere, the light appears yellow. So then, why is moonlight white through the atmosphere?


texts: What would harnessing of gravitational waves look like?

texts: We have harnessed and learned to generate and control sound waves, light waves, water waves, and electromagnetic waves in general.  What would be needed for science to be able to generate and use gravitational waves in the same way? What would be the outcome?


texts:
This is from the NED

What do these three titles mean?

log fb (Jy)
MRK 1014
Log v (HZ)

Why is there LINE,LINE,LINE,LINE,LINE written along the bottom? What does that signify?

What are the dots and lines showing? A range of information? In that chase, what's the relevance of the dot?

Is it like this: The dot is the log v and then the horizontal line shows the range of MRK 1014 recorded for that log v and the vertical line showing the log fv recorded for it?

Why does the log v between 12 - 16.3 have small amounts of log fv and MRK 1014, but the log v between 15 - 19 have no log fv at all, but a large amount of MRK 1014. The same goes for the 2 log v points between 10.4 - 12, except these two have a large amount of log fv but no MRK 1014

Maybe if you could explain the units to me and give me links to further reading it would all become a lot clearer...

Update Okay MRK 1014 is the same of the object. (Doh!) However, what is that titless axis showing?


texts:
Something moving toward or away from us will be contracted in length according to special relativity.

As the M31 galaxy is moving toward us at great speed it's "depth" should appear slightly flattened for us. A sphere moving toward us at the speed of light will appear "pancaked" in lack of a better word.

This was predicted by Lorentz and has been proven in experiments.

Does this also mean that electromagnetic radiation (light) from a flattened star is focused in the direction of motion relative to the observer?


texts: iteration to cover the whole sky with right ascension, declination, angle

texts:
I am sure I get parts of the terminology wrong but if anyone can shed some light in the following:

I understand that for a given right ascension (RA) and declination (DEC), one has defined a ray (half-line) in the sky starting from the center of the earth towards infinity.

Now, if I also provide a angle, say d degrees around that ray, I have essentially defined a cone in the sky. My question is the following:

Which steps (RA_step and DEC_step) should I use in the following loop, as a function of d to ensure that I cover the entire sky and don't leave any patches anywhere?.

for (RA = 0 ; RA <= 360 ; RA += RA_step)
    for (DEC = -90 ; DEC <= 90 ; DEC += DEC_step)
         examine-cone(RA, DEC, d)


texts: Term for the period between planet alignments?

texts: Is there an established astronomical term for the period of time between alignments of pairs of planets?  For example, Mars, Earth, and the Sun roughly align ever 780 Earth days.  In my writing, I've variously call this period the 'alignment cycle' or 'conjunction cycle' or 'opposition cycle', but I'd like to know if there's a correct term for this.


texts:
While practicing for an upcoming examination, I ran into this problem:

Which of the following best describes dust grains in the interstellar medium:

They are a few hundred nanometers in size (size of optical light).
They are much larger than regular dust.
They are primarily composed of helium and hydrogen.
They emit large amounts of blue light.

I did some research and discovered that they scatter blue light, their size ranges form $1 \ldots 100 {\rm \mu m}$ while regular dust is about $1 \ldots 100 \mu{\rm m}$. Also, interstellar medium is composed of helium and hydrogen but that doesn't mean the same about dust. My guess: It is the first answer.
Is this correct? Any assistance much appreciated.


texts: Breakthrough Starshot; could we detect similar probes flying by Earth?

texts:
If extraterrestrial civilizations were sending probes by Earth similar to our plans for Breakthrough Starshot (where the probes are only a few meters in diameter and will be moving at up to 0.2c when they fly by the planet), would we be able to detect them, technologically speaking? And if so, what are the odds we would catch a glimpse of one?

EDIT/ADDITION: An additional question I thought of is whether astronauts would potentially be able to perceive such objects with the naked eye, or would the probes be moving far too fast to register?


texts:
Ok, so I have read about how exoplanets are found using all sorts of genius methods.  I also understand the further an obect in a telescope is the further into the past I am looking.

So given that we are finding exoplantets with methods that go well beyond any practical visual range are we fnding these exoplanets 'in their past' as well? As far as I understand radio telescopes and other methods can't travel faster than light therfore their results are also from past events, is my reasoning in this right?


texts: I live in an area with a lot of light pollution, how can I view the stars without building an observatory?

texts:
I live in an urban area with a lot of light pollution and very little garden space. On a clear night I can see most of the sky and the brighter stars are visible.

I can't build an observatory - I don't have the space (let alone the money required) - so what can I do to minimise the effects of light pollution so I can see more of the fainter stars and planets?


texts: What is the LSST now? Where does LSST end and Vera C. Rubin Observatory begin?

texts:
Space.com's It's Official: Vera Rubin Observatory Named to Honor Dark Matter Scientist says:

"We're here today to focus on the major renaming of the facility after a pioneering astronomer, that is intimately tied to one of the key focus science areas for this project," Ralph Gaume, Director of the National Science Foundation's Division of Astronomical Sciences, said during the event (the 235th meeting of the American Astronomical Society). "I'm pleased, very pleased, beyond how much you all know and may recognize, to today officially rename the LSST observatory as the Vera C. Rubin Observatory."
Gaume's comments were met by applause from the gathered astronomers. Observatory director Steve Kahn and other team members soon donned T-shirts sporting the new name.
The Vera C. Rubin Observatory is a federal project run by the National Science Foundation and the U.S. Department of Energy. Its first 10 years of work will be dedicated entirely to a project now known as the Legacy Survey of Space and Time. "Because we know you're all in love with the four letters LSST, we figured out a way to preserve that," Gaume said.

If I understand correctly, the telescope formerly known as LSST is now the Vera C. Rubin Observatory, but the acronym has been reincarnated.
What exactly is the LSST then? The article says that it is a project known as the Legacy Survey of Space and Time, but what exactly is a "project"? Is it the title of a proposal, or a financial instrument for assigning research funds, or something for business cards and letterhead?

A photo of astronomer Vera Rubin. (Image credit: NOAO/AURA/NSF)


texts: Measurement uncertainty of the computed center of gravity from pixel image

texts:
Establishing the position of celestial bodies (stars, and other objects) is based on imaging technology. Depending on the resolution of the imaging sensor used, an object is identified by a group of neighboring (bright) pixels. These pixels surround the center of gravity of the detected celestial body.

In image processing, the central position of a round object (a sphere) is generally determined by computing the center of gravity of the pixels surrounding the detected object. The weights are the intensities of the pixels in the vicinity of the brightest (central) pixel.

My question: how is the variance of this center-of-gravity estimate being calculated, within astronomy?


texts: Telescope in Sun's gravity lens focus - pointing, gain, distortions

texts:
A telescope located in the gravitational focus of the Sun can use the Sun as a magnifying lens. The focus begins 550 AU away, but maybe a 700 or 1000 AU distance is needed to get rid of disturbances from the Corona, and the focus extends practically indefinately. Here are some slides by Dr. Maccone who has promoted this idea he calls FOCAL:
http://www.spaceroutes.com/astrocon/AstroconVTalks/Maccone-AstroconV.pdf

I intend to ask about the technical design and feasibility of such a project in the Space Exploration SE. Here I rather ask about the scientific value and challenges.

POINTING: The magnification would occur only in the exact direction of the Sun. But since the Sun moves and the background objects magnified move, I suppose that the observed targets would change continuously. Would it even be practically possible to give the telescope a trajectory which keeps it aiming at for example Alpha Centauri? Would there most of the time be nothing in the right direction as the line between the telescope and the Sun sweeps across space, or would there alway be some star or galaxy in sight? Like CMB if nothing else.

GAIN: In the slides linked above, Maccone has calculated the expected gain to 114 dB for infrared wavelengths. How many times "magnification" does this mean? I don't think I understand the units here, I get a ridiculously large number. Can it be explained somewhat intuitively? Would a FOCAL mission be a unique revolution in astronomy, or could similar results be achieved by building an interferometer with interplanetary sized baselines here nearer to the Sun? How does the science value of a gravity lens compare to that of a wide baseline? Are they good for different tasks?

DISTORTIONS: Could the lensed signals be reconstructed thanks to our knowledge of the Sun and measurements of corona activity? If pointed towards a central part of the Milky Way, wouldn't signals come from multiple objects at the same time, some much further away than others? Would the gravity lens of the Sun have bigger problems with distortion than the intergalactic gravity lenses we know of today?

And finally, can any natural strong lensing inside the Milky Way be used today, for example using a globular cluster as a lens?


texts:
Why do the planets have roughly the same inclination with respect to Earth?

Looking at the inclinations, which I got from http://www.astronomynotes.com/tables/tablesb.htm, we see that Mercury has the highest inclination at about 7 degrees with respect to Earth.  Why are there no inclinations at approximately 90 degrees, or even slightly less, like 45 or 30?  My guess is after some long time, gravitational influence causes all the planets to line up.

Question two is then: Do we know of any planetary systems where the planets have high degrees of inclination with respect to each other?


texts: Why wasn't there any oxygen in the beginning of the Earth?

texts:
In the total beginning of the Earth there was CO2, N2 and H2O. So there was O2 but always in bond with Carbon and hydrogen, but what is the reason for that? Is O2 always reacting with free carbon atoms?
The temperature of the Earth was is those beginning days around 1200degree Celsius. Is that also a factor of those bindings?


texts: Does the "amount" of dark energy in the universe change over time, or is it constant?  The dark energy I am asking about is the rate of acceleration of the universe's expansion.


texts: What percentage of a lunar meteor strike is blown back into space?

texts:
Reading a discussion on another website, it was proposed that if a large meteor struck the moon, that a few days later, Earth would receive a meteor shower.

I thought about that and it seemed to me that one of the big unknowns would be what percentage of the impact would reflect back into space, away from the lunar gravity well.

The moon, having no atmosphere, then velocity out, could be pretty close to the velocity in, minus whatever's lost in heat and pressure waves and perhaps phase transition, though that could increase the explosive rebound and maybe other energy changes I'm not considering.

My question is simply, is there a fairly simple formula or estimate of what percentage would blow back into space following a lunar impact.  I'd suspect that it would depend on both the velocity and mass of the meteor and perhaps, composition.  If it was large enough and a direct hit, it could blow the entire moon into space, for example, but I'm thinking more standard asteroid size impacts not giant impacts.

Say, for example, a fairly large impactor like 99942 Apophis, about 27 billion kg, 370 meters in diameter.


texts: spectral index of primordial power spectrum

texts: According to eq (7.62) here, the dimensionless primordial power spectrum takes a power law form: $$\mathcal P(k)=A_s\left(\frac{k}{k_0}\right)^{n_s-1},$$ with $n_s\approx 1$, i.e. $\mathcal P(k)$ is scale invariant. Does this imply a dimensional power spectrum $P(k)=\frac{2\pi^2}{k^3}\mathcal P(k)\propto k^{-3}$? If so, how come other sources, such as eq (18) here, claim that the power spectrum follows Harrison-Zel'dovich form: $P(k)=Ak^n$ with $n=1$?


texts: How do supernova explosions reconcile with light speed constraints?

texts:
I have learned about the various types of supernovae, and both Types Ib and II involve the extremely rapid compaction or explosion of a star's core.

How does the "information" that (respectively) the Chandrasekhar limit or the degeneracy pressure have been reached propagate through the star with enough speed (yet below $c$) that the resulting collapse remains symmetric and happens in a very short time?


texts: Would time go by infinitely fast when crossing the event horizon of a black hole?

texts:
If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon.  If this is correct, would you see the whole universe's future "life" flash before your eyes as you fall in, assuming you could somehow withstand the tremendous forces, and assuming black holes don't evaporate?  If it is correct that black holes evaporate due to Hawking radiation, would you be "transported" forward in time to where the black hole fully evaporates?

This is considering the "alternate" frame of reference from my question: Does matter accumulate just outside the event horizon of a black hole?  In that question, I thought about what happens to matter falling into a black hole from the perspective of someone observing from the outside (e.g. as seen from earth).  Here I am considering the perspective of the thing falling into a black hole.

This also takes into account ideas discussed in: Why does time get slow near a black hole?

Note: This answer to another question provides some insight here too (refer to the last part of the answer): https://astronomy.stackexchange.com/a/3713/1386


texts: About the reference for equation of state

texts: Well, I am a post graduate studying mathematics. Current I am working on the mathematical theory of star(not an expert thought). When I am making some assumption on the equation of state (for a star or sun of course), I simply sum up the radiation pressure, idea gas pressure and degenerate pressure together. In some sense I believe this choice is an appropriate approximate to some model. But when someone ask me why, I am kind of stuck, even thought some of the lecture note posted online supports my guess(for example, http://www.jb.man.ac.uk/~smao/starHtml/equationState.pdf). But I am still wondering, is there any standard text book introducing something like this? I know Chandrasekhar's book has already told something, but it looks like it did not really state that I can simply sum all the pressure together linearly. Thank you in advance.


texts: What observations will be done during the New Horizons flyby of Ultima Thule?

texts:
On New Year's Eve 2018, the New Horizons spacecraft will fly past Ultima Thule (2014 MU69) at about a third of the distance it flew by Pluto some years ago. So it should be able to get some detailed photographs, but the closing speed is rather high. What scientific observations will be done during the flyby?

Is it a case of a couple of snaps, assuming the camera is pointed correctly? Or is there more that can be done? Any other instruments that will get useful data?


texts: Could you use luminosity changes over time to calculate distance?

texts: More specifically: if you know your movement relative to a body, could you use the inverse square law to calculate distance? Since if you are distance $D$ away and you move $d$, $\Delta L \propto (D-d)^2-D^2=2dD-d^2$, I'm assuming you should be able to calculate D linearly.


texts: Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Accurate?

texts:
I'm an artist (and science enthusiast) and I've been trying to find a comprehensive resource that would help me clearly identify likely sky colors (as perceived by human vision) for exoplanets that have atmospheres with a similar chemical composition to Earth. It's trickier than I expected. I've cobbled together several resources, and built what I hope is a decently accurate chart of apparent sun color along with sky color.

UPDATE: I have revised this post and added a new, improved sky chart (below). Science references for sky chart: Ref A, Ref B

How accurate is my chart? Is this a fair representation of sky/sun on alien worlds with heavily Nitrogen/Oxygen atmospheres? In what ways could I improve it?

The chart is not meant to account for things like dust, the look of the sky at sunrise/sunset, or other atmospheric effects. This is meant to be a boilerplate for the baseline look of the sky during the day. However if you'd like to comment on how the sky may change during things like sunset, or with effects like volcanism, I'm game!

Also, I'm more interested in relative color, than getting absolute color 100% right. The image was built in a vector program in RGB mode, so assuming your screen is calibrated in the standard way, we are probably seeing almost the same thing.

Here are the key ideas I've gleamed from my reading that I'm using to build this. I consider most of them tentatively held, and am very open to input:

Earth-like atmospheres would tend to be light to dark blue at the zenith, due to the scattering of low-wavelength blue light. If the planet's sun was very hot, the sky would look a deeper blue, while cooler stars would give the sky a lighter blue to almost white look. When the sun gets to 3000k and below, the sky starts to take on an orange/brown tinge.
Like on earth, the horizon is the lightest in color and the zenith the deepest.
Denser atmospheres would appear brighter (more washed out) and the primary color in the spectrum more "pure" (I'm unsure what the term "pure" means exactly when it comes to optical perception...would it look whiter?). Likewise, thinner atmospheres would be less bright than earth's and the colors more "pure."
With increasing pressure the sky color at the zenith becomes increasingly yellow. In my image this means that an earth-like sky at 10x earth pressure would appear blueish/green near the zenith.
At lower temperatures I'm assuming the sun would appear tinged by the color listed under "star temperature." Otherwise you'll probably only see the star's color when it's near the horizon.
I'm assuming when you get down into K and M class suns, the surroundings on the planet would take on a progressively redder tinge due to the decreasing prevalence of blue wavelength light.
I suspect the gradient of color from horizon to zenith will be steeper/gentler in some atmospheres. I've guessed that the gradient would be more apparent on a high G world (on the right).

References:
Reference #1
Reference #2

Original Chart:


texts: What's the slit and slot on EIS(EUV Imaging Spectrometer) instrument of Hinode?

texts: There are two slits and two slots on EIS, are they just four types of grating?


texts: Can you assume atmosphere height for the purpose of surface pressure calculation?

texts:
Given that by definition of scale height an atmosphere thins by a factor of 1/e^x where x is elevation in terms of scale height multiples (See the table here: Definition of Scale Height), can we assume that the atmosphere is effectively non-existent at the elevation of 6H?

Density at elevation 6H
1/e^6 = ~0.00248 would mean about 0.2% of density at surface level

I know there is no real physical boundary but what I'm looking for is what is the assumed standard for simplifying calculations. Or is it just a bad idea to try and do it this way?

*This is a followup to another question:
How can you determine the initial volume of a planet's atmosphere?


texts: Local Standard Time Meridian - LSTM (Units)

texts:
I was just wondering what units I could scale down to (or or shouldn't) for working out values like Local Solar Times, or Local Time etc...

From the papers I'm looking at, I can't see any mention of SI's or ISO's so I'm a little lost as to what to use as a reference.

Thanks

Ade


texts: Why doesn't the night sky keep on getting brighter and brighter every day?

texts:
I think this should happen because of the below logic.

Consider Day 0 of planet earth. There is no light coming from any surrounding star that has reached our planet and hence its pitch dark.
On Day 1, the light of 1 star reaches us. And once it reaches us it will keep on coming continuously as it's not a pulse, it's just a continuous stream. Let us think that it's a 'string' or 'thread' say which starts from that star and reaches our planet and so we now have this connection.
On Day 2, light from another star reaches us. Again there is 1 more 'thread' which has now connected that star with our planet.
On subsequent days (and in between) more and more such threads get connected with our planet. And unless earlier stars that made this connection with our planet dies, the connections will only grow in number.
And so I would think that the night sky should become brighter and brighter every day.

Why doesn't this happen?


texts: So I was reading this answer about how galaxies are the fastest moving objects in the universe because space is expanding faster than the speed of light. This got me wondering, would it be possible to be 'still' inside of space so that galaxies are moving away from you and others towards you? If so, wouldn't this allow us to travel from place to place in the universe much quicker? I know that this wouldn't be something that's even close to possible today, but I was just curious if such a thing is possible. Thank you.


texts: Spectral Energy Distribution (SED) characteristics of Disk and Elliptical Galaxies

texts:
Quick questions about the spectral energy distributions of galaxies:

What characteristics do we look for in SEDs that provide information on that galaxy's morphology?

For example, in spiral galaxy SEDs, we tend to see an increase in flux density around sub-mm range, which would represent high amounts of dust emission (thermal). Since elliptical galaxies are known to have significantly less gas, this leads us to assume the galaxy is spiral. (there are obviously other factors, but assuming an SED is the only information available). I am particularly interested in elliptical / lenticular galaxies.


texts: Does the radius of your cosmic horizon decrease as velocity increases?

texts: If I was moving at c-1 meters per second, and was observing an object moving 2 m/s away from my starting point as a result of the expansion of the universe rather than classical velocity, would I ever see the light coming from this object? Does that mean that as you move faster, the radius of your cosmic horizon decreases?


texts: What is the application of pseudorandomness in astrophysics?

texts: What are the advantages of using pseudorandom numbers in Monte Carlo simulation instead of random numbers? What are other applications of pseudorandomness in astrophysics? Many thanks.


texts: What's the origin of Clyde Tombaugh's 9-inch Newtonian telescope?

texts:
Astronomer Clyde Tombaugh is shown in a now-famous photograph at the age of 24 with his "homemade" 9-inch Newtonian telescope in this image also shown in Elva R. O'Hara's Clyde W. Tombaugh: Farm Boy Reached for the Stars.
Shown in the photo, the telescope tube is marked with something like

9 INCH AP
NEWTONIAN TELESCOPE
79 INCH FOCUS

along the length of the tube, and illegibly with

OHIO-...

and some other markings in a different, light-toned printing around the circumference of the tube.
According to the linked article:

Tombaugh was born on Feb. 4, 1906, in Streator, Ill. He might have been a typical farm boy but for his one fascination: astronomy. Through the influence of his father and an uncle, the 12-year-old Tombaugh began observing the night sky. He spent hundreds of hours using a 9-inch telescope bought at Sears and Roebuck, learning to methodically identify stars and constellations as well as to observe the movements of the planets.

But on page 243 of Gary Leonard Cameron
Iowa State University 2010 thesis Public skies: telescopes and the popularization of astronomy in the twentieth century it says:

Sears Roebuck had only a few cheap hand-held telescopes for sale in its catalogues from the 1920s to the 1940s. However, by 1961, Sears had a total of nine models of astronomical telescope for sale, all Japanese imports under the Tower label.

Question: So exactly what is the origin of Clyde Tombaugh's 8-inch Newtonian telescope?

Cropped from Elva R. O'Hara's Clyde W. Tombaugh: Farm Boy Reached for the Stars:

The description there says:

Image caption:  Clyde W. Tombaugh shared a homemade telescope with his father and uncle.  Photo courtesy of NMSU Library Archives and Special Collections

Description here says:

The astronomer Clyde Tombaugh, discoverer of Pluto here shown with his homemade 9-inch telescope. Date: circa 1930


texts: How did Johannes Hevelius' long telescope work? Why all the round holes?

texts:
The drawing below, found in Wikimedia and at lib.harvard.edu is of a very long tubed aerial telescope. I believe it is taken from his 1673 work Machinae coelestis.

I've always wondered about the purposes of all those (20+) square blocks with round holes, spaced evenly from one end to the other.

I've included two cropped views from Harvard University, Houghton Library, pga_typ_620_73_451_fig_aa (found here) the first of which I've zoomed and enhanced the contrast both to better appreciate the cute bird, and to try to read the annotations; there seems to be a 772 or 722 next to one of the blocks near the top left.

Question: What was the function of all of these blocks? Is it optical, or mechanical, both, neither? When I was young I thought there were lenses in each one, but I'm assuming now that at least the central circular holes are completely open.

I think it is pretty easy to guess either one, but presumably with such detailed annotation there is text to go along with this image somewhere and it is likely that we can know Hevelius's intention. I just don't know where to read the text.

I believe that the text is here but it's 1,000 pages long and in a language that I can't understand.


texts: "Holes" discovered in the Milky Way Galaxy

texts: Recently I read that Astronomers have noticed a few "holes" in our Galaxy. They are about one million times the mass of our Sun. They are baffled as to what could have caused this and speculate that perhaps it was done by Dark Matter. My question is, what happened with any Stars and Planets in this region ? Would they have been pushed aside or destroyed by this event ?


texts:
I really want to take pictures of Starlink satellites, but I am fairly new to this kind of photography so I have a few questions.

What is the best lense to use and do I need any additional filters?
How do I point my camera at the spot where the satellite will pass through, do I need some kind of tracker for it?

Thank you in advance for your answers, I hope my questions made sence.


texts: Why do aliens have to be carbon based lifeforms?

texts: So correct me if I'm wrong, but when searching for extra terrestrial life, scientists usually search for signs they would expect for life on earth to thrive in. We look for conditions on planets that meet certain criteria e.g close enough to a sun for water, environments for carbon based life forms to exist. I was wondering; why cant an alien be a sulphur based life form that for the sake of argument drinks lead and breathes in chlorine like we do oxygen.


texts: How does a galaxy in Abell 2261 exist without a black hole in the center?

texts:
Here a source says that astronomers looked for a black hole in the brightest galaxy in the cluster Abell 2261, A2261-BCG, but found nothing:

However, X-ray observations from NASA's Chandra X-ray  Observatory and
Hubble Space Telescope have found nothing.

How is it possible for this galaxy to not have a black hole in the center?


texts:
I have High Time Resolution Universe dataset. I have applied de-dispersion on this HTRU dataset, and thereafter I applied the folding function to fold the dataset.

How I can find that the periodicity of the signal using a fast-folding algorithm?


texts: Does the average person in China know the constellations by the traditional Chinese system or Hellenic designation

texts:
As discussed in the International Dunhuang Project website, in traditional Chinese astronomy "the Chinese sky was divided into five great regions or palaces called gong 宫," and "Chinese astronomers identified twenty-eight segments in the sky known as mansions" with "each of the four directions contains seven of the twenty-eight mansions and together with the central region of the sky."

These do not directly correspond to the Hellenistic constellations familiar throughout Western culture.

Has the curriculum in China been nationally standardized so that the traditional Chinese astronomy designations is taught and the Hellenistic constellations is not.


texts: Are stars NML Cygni, UY Scuti, VY Canis Majoris and VV Cephei near the ends of their lives?

texts: Are the four stars mentioned in the title stars that are going to go supernova, or are they stars that are in mid-life, like our Sun, but just happen to be so much larger than our Sun?


texts: Could dark matter form coherent bodies the size of stars or planets?

texts:
Is it possible that dark matter could collapse gravitationally, forming massive bodies similar to stars or planets?
When I say "similar", I don't mean to imply that they would undergo nuclear fusion or have solid surfaces, merely that gravitational collapse could create a much higher density of dark matter within some spherical region of a size similar to a star or planet.
My thought is that if there were, say, some billions of roughly solar-mass collapsed dark matter bodies in this galaxy, and they weren't associated with normal stars, we likely wouldn't be able to detect them. Gravitationally, they'd be part of the background noise; we can't, as far as I know, detect the gravitational influence of individual stars on each other over distances of light-years.
Is this plausible?
(What isn't plausible is that I'm the first person to think of this, so if there's existing research I'd be interested in seeing it.)
I understand that we don't know what dark matter is, and therefore this question might be unanswerable at this time.


texts: Is there an acceptable value for the absolute magnitude of horizontal branch stars?

texts:
I'm writing a test for a gen-ed level astronomy class that I teach, and would just like a good reliable number to use as the basis for a distance calculation question.

And yes, I know that real astronomers use the RR Lyrae stars, but the text were using hasn't introduced variable star standard candles yet.

Looking at some H-R diagrams in textbook the answer seems to be vaguely around magnitude 0, and I'll use that value in the absence of something more definitive.


texts: I just got a 4.5" Newtonian reflector.  The skies haven't been clear where I live lately so I've only been able to use it to see the moon and Jupiter (I was also able to see the Galilean moons).  A friend of mine with a similar telescope says he can see Saturn with its rings and the phases of Venus with his telescope.  I am wondering what else I can see with a telescope my size, specifically which Messier objects I can see with my telescope.  I live in a suburban area near a small urban area and so light pollution is a concern but I have access to darker skies close by.


texts:
What is the farthest orbit that can be achieved around the Earth? How fast is that?

Not the question below:
Feel free to answer in a comment

I ask because of this question: https://worldbuilding.stackexchange.com/questions/104593/can-you-get-a-orbital-mass-driver-to-work-this-way

Is there an orbit exist where 2 objects going in opposite directions can intersect repeatedly with minor course corrections?


texts: Right ascension and sidereal time difference

texts:
I've read quite a few articles about this and watched visualisations but I still struggle to get a mental map of how this works

I understand that on March 21/22 RA 0 is on my meridian at local noon. Ignoring the sunlight, say there is a star also on that meridian at noon, in a month (April 21st) I understand that solar and sidereal time will have a difference of about two hours - does this mean the star will now be about 30 degrees west of the noon meridian?

Also I read that sidereal time means celestial objects are always appearing in the same place in the sky but the sidereal times can be different? I think my main confusion is how sidereal and RA relate.

To summarise I'm looking for more explanation regarding the two hour rule per month. Everything makes sense on the date of the vernal equinox but on say April 21st why is Sirius appearing two hours earlier? I know it's going to be obvious when it hits me :D

Thanks


texts:
There is a sequence of three raw images by Mars Curiosity Rover taken during Sol 3066 (22/03/2022, roughly) that show the same features of Mars' landscape and sky. They are separated by 38 seconds' difference each. This is the first one. This is the last one. Interestingly, the middle one shows a black, oval spot in the low sky, that is not present in the other two:

Just in case some one think I have some obscure intentions, I state I am not an UFO fan, conspiracy theory fan, or anything like that. This is a genuine question.
So, what do you think that might be? Could well be camera noise, or perhaps a satellite (there are a few orbiting Mars), or just some storm debris (there are quite a lot of storms in Mars). Any guesses?

Here's a section blown up so that every pixel produced by the JPGEG is now 5x5 pixels simply to make it easier to see the artifact:

Update: Ok, it seems there are lots of similar images. For instance, an image from more recent Sol (3067) also shows a black spot in the sky. An image a bit earlier shows a white spot (from comment below). Here there is what it looks like a meteorite with a trail.


texts: Is Earth's climate significantly different when the one solstice occurs at aphelion rather than the other?

texts: I mean different in some kind of significantlly non-symmetrical way, such as it being much colder during one period rather than the other? Let's assume other factors, such as tilt due to nutation and variation in the Sun, are the same.